In the art of papermaking, a well known incentive includes increasing the amount of filler in the final paper product. This incentive can be driven by both a shortage of pulp for papermaking and pulp prices.
One method for increasing the filler content in paper is the use of chemical retention aids. Retention aids typically permit an increase in filler content by. modifying the bonding relationship between the fiber in the pulp and the filler. Typically, filler particles are much smaller than most pulp fibers and are not effectively retained by filtration through the pulp matte as it forms on a papermaking machine. Retention aids include cationic starches, charge-biasing polymeric types such as amine or quaternary ammonium groups and polymeric bridging agents such as ionic, cationic or anionic polymers. Often times, different retention aids are added for synergistic improvements such as using both a low molecular weight cationic polymer and a high molecular weight anionic polymer.
FIG. 1 shows a typical prior art papermaking method using retention aids to increase filler retention. A portion of an overall papermaking apparatus is designated by the reference numeral 10 and is seen to include a thick stock machine chest 1, a fan pump 3, a head box 5, white water silo 7 and a papermaking machine 9. It should be understood that since this apparatus is well known in the art, a further detailed description of the components upstream of the machine chest 1 and downstream of the papermaking machine 9 are not included or deemed necessary for understanding of the invention.
In use, a filler 11 is added to a pulp slurry 13 to form a filler-containing pulp slurry 15 or the filler 11 is fed directly to the thick stock machine stock 1 or fan pump 3. The thick stock 17 is then fed to the fan pump 3. The white water recycle 19 from the silo 7 is also fed to the fan pump 3 to form the thin stock 21. The retention aid 23 is then added to the thin stock upstream of the head box 5 to increase retention of the filler during papermaking, typically past the screens and cleaners.
In a similar prior art system as described in the publication entitled "Three Developments at Wolvercote Paper Mill" by M. C. Riddell et al., Paper Technology and Industry, April, 1976, pages 76-80, the retention aid is a high molecular weight synthetic polymer with a specific charge density and the filler is a clay or calcium carbonate. Addition of the retention aid to the filler-containing pulp slurry results in preflocculation of the filler. Adding the retention aid at the head box minimizes shearing of the preflocculated filler and possible lowering of filler retention.
One problem with these types of papermaking systems occurs when using a pulp containing high amounts of anionic trash. When adding a cationic retention aid to a pulp containing high amounts of anionic trash, the cationic retention aid tends to become neutralized by the anionic trash present in the pulp. With this neutralization of the retention aid, the filler retention during papermaking is reduced.
A prior art solution to this problem has been the addition of more cationic retention aid. More specifically, a cationic coagulant is added to the pulp slurry upstream to the filling addition followed by addition of a retention aid at the head box after filler addition. This approach is not only expensive, with as much as $50.00 of chemical costs being added to the cost of paper, but can also be difficult to run on the paper machine and can lead to significant down time on the machine without very close machine monitoring. Moreover, with the increased operating costs, approaches of this type are usually limited to a special grade of paper wherein the manufacturer can recoup the increased operating costs through the value of the final paper product.
Another problem with the prior art system discussed above wherein preflocculation of the filler occurs due to the retention aid addition is the formation of large agglomerates which are mechanically entrapped in the paper web. Although filler retention as high as 99% can be obtained, this approach leads to very large particles which create a mottled appearance on the sheet and could also lead to dusting problems.
One solution to the creation of a mottled appearance on the sheet is applying shear to the preflocculated filler particles to reduce their size and effect on sheet appearance. However, this shearing process is difficult to control and also greatly reduces the retention of the filler material.
Referring now to FIG. 2, the effective shear on the retention of a preflocked filler clay and a standard retention aid treatment is shown. As expected, the preflocked filler treatment shows higher ash retention percentages than the standard treatment at low shear rates. However, increasing the shear rate to improve the surface appearance adversely effects the percent ash retained of the preflocked filler material. In fact, by the time sheet appearance is acceptable by increasing the shear rate to around 1000 RPM, the retention of filler is only marginally better for the preflocked filler than the standard treatment. However, if sheet appearance is not a problem, preflocked fillers can produce a significant cost savings over the standard treatment. However, the use of preflocked fillers is limited when seeking acceptable sheet appearance.
In view of the disadvantages noted above in the prior art, a need has developed to provide an improved method of maintaining or increasing filler retention while using lower amounts of retention aids and maintaining acceptable surface appearance.
In response to this need, the present invention solves the problems of the prior art discussed above by providing both a method and apparatus for improved filler retention in papermaking which not only reduces the amount of retention aids and cationic coagulants used but also maintains acceptable surface appearance in the paper product and provides high levels of filler retention.